add missing scrape-examples argument

[syphar]

I regularly recommend this cargo command to docs.rs users so they can test the output, so thanks for this!

After digging into a size difference in the actual docs.rs output and cargo docs-rs output I found this argument is missing.

See also rust-lang/docs.rs#1954 where it was added.

[dtolnay]

Thanks for the PR. I would prefer not to enable this until after rust-lang/cargo#15155 is addressed.